The present invention relates generally to well logging systems and methods and, more particularly, to dielectric well logging systems and methods.
Well logging systems including a tool comprising a transmitter and receivers and providing a measurement of the dielectric properties of earth formations surrounding a borehole are known. In one such system, see, e.g. U.S. Pat. No. 4,185,238 issued Jan. 22, 1980 to Huchital et al. and assigned to the assignee of the present invention, electromagentic energy is generated at a first location in the borehole and the relative attenuation of the electromagnetic energy is detected at a second location in the borehole. The detection is performed using a first two receiver differential arrangement. The relative phase of the electromagnetic energy is detected at a third location in the borehole by a second two receiver differential arrangement. Means are provided for determining the dielectric constant and/or the conductivity of the formations as a function of the detected relative attenuation and relative phase.
Due to the high temperature variations present in borehole well logging, considerable drift can occur in known phase and amplitude measuring devices. Solutions have been advanced to resolve that problem. For example, as described in U.S. Pat. No. 4,100,483, issued July 11, 1978 to Thompson et al., phase measurement circuits are provided at the surface, away from the borehole temperature effects, rather than in the downhole tool. In this manner the effects of temperature variations allegedly are reduced.
Providing phase measurement circuits at the surface is disadvantageous in some ways. Data transmission complexity is increased. Moreover, the transmission medium itself, i.e. a cable, is subjected to temperature variations encountered in the borehole. Since the volume of information transmitted to the surface is greater, the task of reducing the adverse effects of these temperature variations is rendered more difficult. Thus in processing the receiver signals to extract therefrom useful information regarding phase and amplitude differences in an efficient and useful manner, the use of measurement circuitry at the surface is disadvantageous.